The present invention relates generally to compression and expansion systems for transmitting signals with reduced noise, and more particularly to a compression and expansion system capable of enlarging the dynamic range of the high-frequency range of a transmitted signal.
A system called the Dolby S/N stretcher has been known in the prior art as a compression and expansion system for reducing noise. Another known system is the so-called ANRS system which has solved the problems accompanying the Dolby system. The ANRS system is disclosed in several publications such as, for example, the specification of U.S. Pat. No. 3,757,254.
The compressing part of each of these systems has a frequency characteristic wherein, as the level of the input signal becomes higher, the frequency characteristic of the output level in all frequencies becomes flatter. As the level of the input signal becomes lower, the level of the high-frequency range is increased in comparison with the level of the low-frequency range. In the expanding part of each of these systems, the level is to complement attenuated the above mentioned level increase, and the transmission characteristic in the entire system becomes flat. Moreover, the noise in the high-frequency range is reduced.
However, the saturation level of the input-output characteristic decreases with increasing signal frequency responsive to the relationship between the level of the input recording signal and the level of the output reproduced signal in a magnetic tape recording. Consequently, as the signal frequency becomes higher, the characteristic becomes saturated at a level which is not very high. The linearity of the proportional relationship between the input and output is lost.
As a consequence, each of the above mentioned compression and expansion systems has a dynamic range which cannot be large, particularly in the high-frequency range.